The present invention relates to a sheet retainer to be affixed to a punching tool for punching fine through holes such as for wiring on a ceramic green sheet to be used for manufacturing a high-density wiring circuit board and the like and a punching apparatus provided with such a sheet retainer.
Numerically controlled die punching apparatus and the like are in use for punching a ceramic green sheet (hereinafter greensheet).
A punching apparatus is provided with a vertically movable punching tool on the tip of which a fine shaft-like or needle-like work pin with a diameter of several mm to less than 1 mm is provided corresponding to the through hole to be punched. Through holes are formed by punching a green sheet with the work pin through the vertical movement of the punching tool.
It happens that the green sheet is pulled; up attached to the work pin that has just punched a green sheet when pulled out from the through hole. This causes problems of deforming the green sheet, cracking it, or deforming the through hole. Also, when the work pin punches the green sheet, the sheet material that comes in contact with the work pin at times slides off or moves, thus disturbing the formation of a through hole at a correct position and with a correct shape.
In order to prevent such lifting and moving of a green sheet, a proposal has been made to affix a sheet retainer called xe2x80x9cstripperxe2x80x9d for example, to the punching tool.
A sheet retainer is usually composed of hard rubber and is cylindrical in shape with approximately the same diameter encompassing the outer periphery of the work pin when affixed to a punching tool. When the work pin is moved down to punch a green sheet, the tip of the cylindrical sheet retainer comes in contact with the surface of the green sheet. As the work pin is further moved down, the sheet retainer is elastically deformed and keeps pressing the green sheet with its restoring force. During manufacturing the work pin is moved up after punching a through hole. As the elastically deformed sheet retainer keeps pressing the green sheet from above until the work pin has been sufficiently removed from the surface of the green sheet, a lifting of the green sheet is prevented.
A practical construction of a sheet retainer is disclosed in Japanese Laid-Open Patent Publication No. H09-205265. The sheet retainer as described in this document is a rubber-made cover called stripper. The entire body is cylindrical in shape with a relatively thin uniform thickness with only the lower end portion (the portion which comes in contact with the surface of a green sheet) slightly narrowing toward the center. This was for the purpose of preventing lateral slide and vertical moving of the green sheet.
Previous sheet retainers suffer problems :of not being able to fully prevent lifting of a green sheet because of an inappropriate pressure or a too weak pressure on the green sheet, or conversely, deforming or damaging the green sheet because of a too strong pressure.
This is due to variations of the pressure or pressing force against the green sheet which depends on the hardness of the rubber material used as base material of the sheet retainer and the shape of the sheet retainer.
For instance, when a sheet retainer made of a rubber material which is too hard is overly pressed against a green sheet, it may cause a crack in or a deformation of the green sheet. When the rubber material is too soft, the pressing force against the green sheet will be insufficient and securing of the sheet retainer on the punching tool tends to be insufficient.
An object of the present invention is to eliminate the above described problems associated with conventional sheet retainers and to enable one to surely prevent moving, deformation, or damage of a green sheet during punching work.
A sheet retainer for punching a ceramic green sheet in accordance with the present invention is a sheet retainer to be affixed to a punching tool having a cutter section for punching a through hole on a ceramic green sheet. The sheet retainer is made of a rubber material having a hardness of 50 to 60 degrees (JIS K 6301-1995) and includes a support portion to be engaged; to the punching tool, a contact portion which comes in contact with a green sheet, and a housing hole for housing the cutter section. The support portion is disposed on one end in axial direction with a relatively large outer diameter and thick-walled in the radial direction while the contact portion is disposed on the other end in the axial direction with a relatively small outer diameter and thin-walled in the radial direction.
In the present invention, a ceramic green sheet to be used for fabrication of an ordinary ceramic multi-layer substrate can be used. As the ceramic material, alumina, glass ceramic and the like are used. A green sheet is a non-hardened sheet-form material before being hardened by firing.
The punching apparatus of the present invention includes a base for laying a green sheet, a punching tool having a cutter section for punching and being movable toward the base, and a sheet retainer to be affixed to the punching tool, the sheet retainer having the above described features.
The punching apparatus is provided with mechanical structures such as a base for laying a green sheet, a punching tool attachable/detachable mechanism called a punch pin, for example, and an actuating mechanism for moving the punching tool toward the base.
The punching tool has a mechanism for moving to or moving away from the punching apparatus. The tip of the punching, tool forms a thin axial or needle-like cutter section corresponding to the diameter of the through hole to be punched.
The punching tool has a structure for affixing a sheet retainer in an attachable/detachable manner. As a structure for affixing the sheet retainer, a ridge or a groove, or an uneven shape such as pits and projections is provided around the punching tool.
The sheet retainer of the present invention is made of an elastic material such as rubber and is affixed to the punching tool in an attachable/detachable manner.
As the elastic material, synthetic rubber such as silicone rubber and synthetic resin may be used in addition to natural rubber. The hardness of the elastic material may be adjusted such that pressing of a green sheet can be adequately performed. Depending on the dimensions and shape of the sheet retainer, a material with hardness ranging between 50 to 60 degrees is used. Here, the hardness represents the value measured by using a spring type hardness tester as specified by JIS K 6301-1995 for measurement of hardness of vulcanized rubber. Also, a material with a high shape-restoring factor is preferable, and a material with a shape-restoring factor of substantially 100% toward the load applied while in use is preferable.
The shape of the sheet retainer is basically roughly cylindrical as a whole, and its one end in the axial direction has a support portion into which a punching tool can be engaged and the other end in the axial direction has a contact portion that comes in contact with a green sheet. A housing hole for housing the cutter section extends axially from the contact portion.
When the outer diameter of the support portion and the outer diameter of the contact portion are compared, the outer diameter of the support portion is made larger. Also, when the wall thickness of the cylindrical sheet retainer is measured in radial direction for the support portion and for the contact portion, it is greater for the support portion and smaller for the contact portion. The thick-walled support portion is hard to be deformed and the thin-walled contact portion is easy to be deformed.
The outer diameter may be tapered from the support portion toward the contact portion. The diameter may be changed by small steps. A combination of tapering and stepping may also be allowed.
The ratio between the wall thickness T1 in the radial direction at the contact portion and the maximum wall thickness Tmax in the radial direction may be kept at T1/Tmax=0.1 to 0.69. The maximum value T max of the wall thickness generally is at or in proximity to the support portion. The greater Tmax becomes, the greater the rigidity on the side of the support portion becomes, and the smaller T1 becomes, the easier elastic deformation on the side of the contact portion becomes. However, when T1/Tmax is too small making deformation on the side of the contact portion too easy, the pressing ability against the green sheet becomes insufficient.